# Diagnostic Test Calculator

This calculator can determine diagnostic test characteristics (sensitivity, specificity, likelihood ratios) and/or determine the post-test probability of disease given given the pre-test probability and test characteristics. Given sample sizes, confidence intervals are also computed.

Fill out one of the sections below on the left, and then click on the 'Compute' button. Sections you don't fill out will be computed for you, and the nomogram on the right will display the probability that a patient has the disease after a positive or negative test.

Numbers of patients with and without the disease who test positive and negative:

Disease present Disease absent Total
Test positive 61
Test negative 223
Total 51 233 284

or
disease prevalence, test sensitivity, and test specificity (and, optionally, sample size):

 Prevalence (e.g. 0.10): Sensitivity (e.g. 0.80): Specificity (e.g. 0.80): Total sample size:

or
disease prevalence, positive likelihood ratio, and negative likelihood ratio (and, optionally, sample size):

 Prevalence (e.g. 0.10): +LR (e.g. 4): -LR (e.g. 0.01): Total sample size:

Optional information:

 Your local prevalence (e.g. 0.10): Probability of disease at or above which you would be comfortable treating with no further testing (e.g. 0.8) Probability of disease at or below which you would be comfortable managing with no futher treatment or testing (e.g. 0.25) Probability of disease at or above which you would treat (comfortable or not) and below which you would not treat, if there were no further options (e.g. 0.4)  Prior probability (odds): 18% (0.2) POSITIVE TEST: Positive Likelihood ratio: 8.70 95% confidence interval: [5.65,13] Posterior probability (odds): 66% (1.9) 95% confidence interval: [55%,74%] (~ 1 in 1.5 with positive test are sick) NEGATIVE TEST: Negative Likelihood ratio: 0.24 95% confidence interval: [0.14,0.40] Posterior probability (odds): 5% (0.1) 95% confidence interval: [3%,8%] (~ 1 in 1.1 with negative test are well) Odds = Probability / (1-Probability) +LR = Sensitivity / (1 - Specificity) -LR = (1 - Sensitivity) / Specificity Posterior Odds = Prior Odds x LR
Embeddable URL: http://araw.mede.uic.edu/cgi-bin/testcalc.pl?DT=40&Dt=11&dT=21&dt=212&2x2=Compute
Diagnostic test calculator (version 2010042101). Copyright (c) 2002-2006 by Alan Schwartz <alansz@uic.edu>. This calculator is Free Software, available under the Clarified Artistic License.